audio_device: Move sources into platform-conditions.
This should solve a problem discovered when converting from GYP to
other project formats, where the source files weren't included correctly
for each platform.
Two other targets in WebRTC have similar source files, which are correctly
generated for each platform:
* video_render_module_internal_impl
* video_capture_module_internal_impl
They both list the sources as it's changed to in this CL.
NOTRY=True
Review URL: https://codereview.webrtc.org/1536923003
Cr-Commit-Position: refs/heads/master@{#11083}